They are heavy for their gauge, at least the more common 12-16-20 series, but as Michael McIntosh once said I believe: "Hell for stout"--
There are three disciplines that fascinate me to no end, even somedays make me wish I had attended college: metallurgy, classical music and architecture--My SWAG as a retired welder re: M21 blue wear, ditto M12 in receiver areas might be a slight variance in the AISI 4000 range of steels- aka- nickel, chromium and molybdenum-- the M21 barrels, as also the post 1930's M12 were made from WPS- which I believe to be AISI 4140 Chrome/Moly with a 40carbon point range- but I suspect the receivers were forged from 4340 Nickel/Chrome/Moly with again 40 point carbon content--
The additional nickel content in the 4340 steels MAY account for that blueing wear, especially if the barrels were rust blued and the receivers, which had no lug brazing or soldered ribs as do the barrel assemblies, were dip blued or blued with the Du-Lite process developed by "Uncle Dupey" down in Wilmington DE back in that era.
I have always felt that Mr. Olin (no disrespect intended here) may have 'stacked the deck' in the famous Blue Pill proof test that the M21 "aced" and to compare a 12 M21 with a Churchill XXV proofed to light loads in a 2.5" shell would be like pitting Arnold Stang against "Mr. T" ina toughman contest.
